Laboratory measurements of Aporrectodea caliginosa earthworm counts using qPCR in agricultural soils

Abstract

Traditionally, the abundance of earthworms in soil is perceived as a good biological indicator of soil health. While the direct relationship between earthworms and soil health is perhaps uncertain, it is well known that earthworms do provide benefits to the soil. The conventional method of determining the abundance of earthworms is a physical count within a spade square of sampled soil, which is a manual and timeconsuming
process, and where speciation is desired, requires technical expertise. Earthworm abundances vary spatially and hence the values derived will be
dependent on the sites selected for measurement. Hill Labs in conjunction with AgResearch have developed a quantitative PCR (qPCR) test to determine the concentration of environmental DNA (eDNA) of the earthworm Aporrectodea caliginosa. A. caliginosa, is the most abundant earthworm in agricultural New Zealand. The eDNA contained within routine (7.5 cm transect cores, 38°C dried and < 2 mm ground) agronomic soil samples, was correlated to abundance. The field calibration was highly correlated with a log equation having an R2 of 0.68, and the remaining
variation largely explainable by the precision of the measurements. While eDNA appears to degrade in the field-moist samples post collection, the eDNA is stable once the soil samples are dried and ground within the lab. The analysis is run in triplicate to account for inherent variation and potential for ‘spikes’ of genomic DNA within the subsampling process. While there is evidence that a single result is as precise as the fieldsampling
technique, we believe testing in triplicate and using the average value (excluding outliers), provides a result that is more reflective of the earthworm population. We believe this method is overall fit-forpurpose for determining the state of earthworms in New Zealand.